Looking for advice on buying large collection

[King44]

Let me start off by apologizing for all the fluff in the beginning here...feel free to skip to the last paragraph...

I took a number of years off from cards because I didn't like the direction modern cards were going, but I'm getting back into it with a focus on vintage cards as well as iconic cards from all eras. I consider myself to be equal parts collector and seller. Currently I sell clothes (mostly vintage) on ebay. In the last few weeks I've been going over a plan to have a hybrid business of clothes and cards. So, that's the backstory...thanks for hanging in there

Well, as it happens I came across a guy a couple hours from me that is selling his vintage collection. Lots of 50s and 60s, cards are in nice condition (lots of 5s-7s). It's about 50 binders of mostly complete or complete sets. Who knows, maybe 10,000 additional cards in boxes. I'm a bit in over my head with figuring out a value on that many cards. I'm actually having a harder time than I thought finding good information online about the best process to figure value on collections.

So what's the best way to value a vintage card collection? I figure I start by looking for the key cards and add those up and go from there? What is the most efficient method for me to come up with a fair price for both of us? This would be a big purchase for me as I don't usually buy this much quantity or value. Any help is appreciated and I understand a post like this could be annoying for some doing this for a long time, but I'm just trying to get traction here starting my journey. Thanks all...